1. Field of the Invention
The present invention relates to observation optical systems such as telescopes and binoculars and, more particularly, to an observation optical system having the zoom function of varying the magnification of an image being observed.
2. Description of Related Art
Mainstream conventional zoom binoculars perform zooming by a variable magnification eyepiece lens having a moving lens unit. FIG. 36 is a schematic diagram showing one example of conventional zoom binoculars, in which, for the purpose of simplicity, only one of the right and left optical systems of the binocular is illustrated.
In FIG. 36, reference numeral 100 denotes an objective lens of positive refractive power. An erect prism P is provided for erecting, and laterally reversing an object image OBI formed mainly by the objective lens 100. A variable magnification eyepiece lens 200 comprises a first lens unit 201 of negative refractive power, a second lens unit 202 of positive refractive power and a third lens unit 203 of positive refractive power. The object image OBI is formed at a position before (on the object side of) the second lens unit 202 through the objective lens 100, the erect prism P and the first lens unit 201 of the eyepiece lens 200.
In the binocular of such construction, the first lens unit 201 of the variable magnification eyepiece lens 200 is made axially movable for varying the image magnification, and to compensate for the image shift, or to correct the diopter, the second lens unit 202 is made to axially move.
Such a variable magnification eyepiece lens 200 is favorable to realize zoom binoculars relatively easily. Up to now, therefore, this arrangement has been very popularly employed.
However, the use of the above-described construction and arrangement for the variable magnification eyepiece lens in the zoom binoculars results in as narrow an apparent angle of view as 35-40 degrees at the low magnifications. To widen the field coverage, there is a need to increase the diameter of the second lens unit 202 and the bulk of the erect prism P. For this reason, it is difficult for the prior art arrangement to simultaneously fulfill the requirements of increasing the apparent field angle and of minimizing the bulk and size of the binocular.
An object of the invention is to provide an observation optical system capable of increasing the field angle at the low magnification (in the wide-angle end) without having to increase the bulk and size of an apparatus having the observation optical system.
To attain the above object, in accordance with a first aspect of the invention, there is provided an observation optical system, which comprises, in order from an object side, an objective lens having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, an erect optical system, and an eyepiece lens of positive refractive power for observing an object image formed through the objective lens, wherein an air separation between the first lens unit and the second lens unit and an air separation between the second lens unit and the third lens unit are made variable to effect variation of magnification.
In accordance with a second aspect of the invention, there is provided an observation optical system, which comprises, in order from an object side, an objective lens having a front lens unit of positive refractive power, a variator lens unit and a compensator lens unit, an erect optical system, and an eyepiece lens of positive refractive power for observing an object image formed through the objective lens, wherein the following condition is satisfied:
0.7 less than xcex2t less than 1.4 
where xcex2t is an overall lateral magnification in a telephoto end of the objective lens excluding the front lens unit.
In accordance with a third aspect of the invention, there is provided an optical apparatus comprising the observation optical system according to one of the first and second aspects described above.
In accordance with a fourth aspect of the invention, there is provided a binocular, which comprises, in order from an object side, a pair of objective lenses each having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, a pair of erect optical systems, and a pair of eyepiece lenses of positive refractive power for observing a pair of object images respectively formed through the pair of objective lenses, wherein in each of the pair of objective lenses, an air separation between the first lens unit and the second lens unit and an air separation between the second lens unit and the third lens unit are made variable to effect variation of magnification.
In accordance with a fifth aspect of the invention, there is provided a binocular, which comprises, in order from an object side, a pair of objective lenses of variable focal length, a pair of erect optical systems each having an entering optical axis and an exiting optical axis deviating from each other, and a pair of eyepiece lenses, wherein interpupillary adjustment is performed by rotating the erect optical systems and the eyepiece lenses around respective optical axes, as rotation axes, of the objective lenses.
The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.